Image: Flag of the Gentleman Pensioners waving

Text: I’m the new captain of the Gentleman Pensioners and I feel you will be best employed as a Gentleman. The corps are traditionally bodyguards of the King and require an oath, but I can get you a type of Catholic exception.

This is the oldest insignia I could find which would probably be in use at the time. http://wiki.bcw-project.org/_media/royalist/horse-regiments/gentlemen_pensioners_wavy.png
Currently the Gentleman Pensioners are https://en.wikipedia.org/wiki/Honourable_Corps_of_Gentlemen_at_Arms and by the time in the story, they were just ceremonial. They just stand by the Monarch when they’re doing state things or ceremonies.

They were originally bodyguards and took part in a few battles, but this would have just been a source of money for Thomas Percy.